Mid-infrared continua via spectral broadening and difference frequency generation in a nanophotonic lithium niobate waveguide

This paper presents the generation of a broadband mid-infrared (MIR) continuum through supercontinuum generation (SC) and difference frequency generation (DFG) in a nanophotonic lithium niobate (LiNbO₃) waveguide. The system uses an erbium-doped fiber laser operating at telecom wavelengths to pump the waveguide, where spectral broadening and MIR light generation occur in two stages: third-order nonlinear SC followed by efficient intrapulse DFG.

Key Findings:

1. Dual-Stage Waveguide Design:

• The device utilizes a two-stage waveguide, where the first stage induces supercontinuum generation (SC) via third-order nonlinear spectral broadening, and the second stage performs intrapulse difference frequency generation (IDFG) with periodic poling.

• The resulting MIR light covers the spectral range from 3200 nm to 4800 nm, with further harmonic generation extending the spectrum into the visible and ultraviolet domains. The total spectral bandwidth achieved spans from 350 nm to 4800 nm.

2. MIR Light Generation:

• The generated MIR spectrum is achieved using a pump source with sub-100 fs pulses at 200 pJ energy. The SC stage uses anomalous dispersion in the waveguide to generate the broad spectrum, while the IDFG stage employs quasi-phase matching (QPM) to efficiently generate the MIR light.

• The MIR output is measured, demonstrating efficient light conversion with a modest power output of 1.22 mW.

3. Waveguide Design and Fabrication:

• The waveguide is fabricated from LiNbO₃ on insulator (LNOI), with dimensions optimized for both SC generation and efficient DFG. The waveguide has a width ranging from 1.2 µm to 3 µm to balance spectral broadening and nonlinearity. The waveguide is also periodically poled to enable QPM for the DFG stage.

• Group velocity dispersion (GVD) is carefully controlled to maximize SC generation and optimize the efficiency of IDFG.

4. Experimental Results:

• The experiment shows that the poled waveguide generates significant MIR light from 3 µm to 4.8 µm, with up to 20 dB higher output compared to the unpoled waveguide. The output power and spectral characteristics are measured, with a roll-off in the spectrum beyond 4.8 µm due to absorption from the silica cladding.

• The system demonstrates a modulation depth of 43% and a bandwidth of 5 MHz, showing the potential for integrated MIR photonic sources.

5. Applications and Future Work:

• This work provides a compact, integrated MIR light source for applications in molecular fingerprinting, environmental monitoring, and precision frequency metrology.

• The paper also notes that the system's output could act as a frequency comb with zero carrier-envelope offset frequency, offering high precision for time-domain and frequency-domain applications.

• Future research is needed to clarify the spectral structure of the generated MIR light and improve fabrication precision to enhance efficiency.

Conclusion:

This paper demonstrates a dual-stage LiNbO₃ waveguide for broadband mid-infrared generation via SC and IDFG, providing a compact and efficient platform for generating coherent MIR light from a telecom-wavelength pump source. This approach opens up new possibilities for integrated photonics in the MIR range, with potential applications in quantum spectroscopy, chemical sensing, and fundamental physics studies.